JPH09180234A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high utilization efficiency stable to temp. by using a specific liquid crystal compsn. having a large difference between an ordinary index and extraordinary index as a nematic liquid crystal phase. SOLUTION: A glass substrate 3 having a polyimide oriented film 5 formed on a surface in contact with the liquid crystal compsn. 4 is adhered to a glass substrate 1 along the stripe direction of the grid-like rugged parts 2 of the substrate. For this purpose, an epoxy resin 8 is applied on the peripheral parts of the glass substrate 1 and the glass substrate 3 is placed thereon and pressed. At this time, a region not coated with the epoxy resin 8 is left as an aperture for injecting the liquid crystal compsn. 4. The liquid crystal compsn. 4 of 0.21 to 0.35 in the difference Δn between the ordinary index and extraordinary index at 20 deg.C and 589nm light wavelength and 80 deg.C in the phase transition temp. from a nematic phase to an isotropic phase is thereafter injected and sealed. Further, a phase difference film 6 and a glass substrate 7 are formed and in addition, light incident and exit surfaces 11, 12 are provided with antireflection films 13. As a result, the high transmittance and utilization efficiency stable to temp. are obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device for writing and reading optical information on optical disks such as CDs (compact disks), CD-ROMs and video disks and magneto-optical disks. About.

[0002]

2. Description of the Related Art Conventionally, as an optical head device for writing optical information on an optical disk or a magneto-optical disk or reading optical information, a signal light reflected from a recording surface of the disk is guided to a detection unit (beam). A prism type beam splitter as an optical component for splitting;
It has been known to use a diffraction grating or a hologram element.

Conventionally, in a diffraction grating or hologram element for an optical head device, a rectangular grating (relief type) having a rectangular cross section is formed on a glass or plastic substrate by a dry etching method or an injection molding method. It was diffracted and had a beam splitting function.

Further, in order to improve the light utilization efficiency of an isotropic diffraction grating having a light utilization efficiency of about 10%, it is conceivable to use polarized light. When trying to use polarized light, a prism type beam splitter is combined with a λ / 4 plate to increase the efficiency of going (direction from the light source to the recording surface) and returning (direction from the recording surface to the detection section) to improve the round trip efficiency. There was a way to raise it.

However, the prism type polarizing beam splitter is expensive, and other methods have been sought. As one method, a method of using a flat plate of birefringent crystal such as LiNbO ₃ and forming an anisotropic diffraction grating on the surface to have deflection selectivity is known. However, the birefringent crystal itself is expensive, and application to the consumer field is difficult. Further, since the lattice is usually formed by the proton exchange method, it is difficult to form a lattice having a fine pitch.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% in the forward direction (direction from the light source to the recording surface) and 20% utilization efficiency in the return direction (direction from the recording surface to the detection portion). Since it is about 10%, the limit is about 10% in both directions.

On the other hand, an optical head device using a hologram (diffraction element) having a high light utilization efficiency by forming a grid-like uneven portion on a transparent substrate and filling the liquid crystal composition therein is the subject of the present application. Suggested by people.

However, the liquid crystal composition materials that are industrially and commonly used in liquid crystal displays have the drawbacks that the anisotropy of the refractive index is small and the operating temperature range is narrow. If the anisotropy of the refractive index is small, it is necessary to deepen the grating depth in order to obtain the desired diffraction efficiency, and the processing itself is extremely difficult. In addition, there is a problem that processing costs a lot.

Further, if the phase transition temperature on the high temperature side is low, even if liquid crystallinity (optical anisotropy) is exhibited at the maximum temperature of 60 to 70 ° C. used as an optical head device, the diffraction efficiency is greatly reduced at around room temperature. However, it was not suitable for practical use. There was also a problem with reliability at high temperatures.

Further, when the phase transition temperature on the low temperature side is high, there is a problem in that even if liquid crystallinity is exhibited near 0 ° C. which is the lowest temperature used as an optical head device, the diffraction efficiency is greatly reduced near room temperature, which is a practical problem. Was not suitable for. There was also a problem with reliability at low temperatures.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an optical head device having a high light utilization efficiency in a wide temperature range.

[0012]

The present invention provides an optical head device for writing information and / or reading information by irradiating an optical recording medium with light from a light source through a diffraction element, wherein the diffraction element is A transparent substrate having a concave-convex portion in a grid pattern, and the concave-convex portion is filled with a liquid crystal composition having optical anisotropy. 20 ° C, light wavelength 58
A nematic liquid crystal composition having a difference Δn between the ordinary refractive index and the extraordinary refractive index at 9 nm of 0.21 to 0.35 and a phase transition temperature from a nematic phase to an isotropic phase of 80 ° C. or higher. An optical head device is provided.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the irregularities may be optically isotropic or anisotropic. When the uneven portion is optically isotropic, a glass substrate having a refractive index of about 1.5, a plastic substrate, or the like is used as the transparent substrate, and the transparent substrate is directly formed with the uneven portion. It can be used and is preferable.

In general, the ordinary refractive index of the liquid crystal composition is lower than the extraordinary refractive index, and the ordinary refractive index is often around 1.5, so that the refractive index of the substrate and the ordinary refractive index of the liquid crystal composition are equal. Is easy to realize and is preferable.

It is considered that the major axis direction of the liquid crystal molecules is aligned parallel to the stripe direction (longitudinal direction) of the grid-shaped irregularities. Therefore, when viewed in FIG. 1, the ordinary refractive index of the liquid crystal composition corresponds to the light (P-wave) polarized in the direction parallel to the paper surface, and the ordinary refractive index and the transparent substrate have substantially the same refractive index. Does not work as a grid. On the other hand, the extraordinary refractive index of the liquid crystal corresponds to the light (S wave) polarized perpendicular to the paper surface, and the extraordinary refractive index of the liquid crystal and the refractive index of the substrate are different and function as a diffraction grating.

Therefore, the optically anisotropic diffraction grating of the present invention is optically transparent for P waves and functions as a diffraction grating for S waves.

On the transparent substrate provided with the optical anisotropic diffraction grating (on the side of the optical recording medium), a retardation film (λ /
When 4 plates are laminated, the P wave incident from the lower side (light source side) is transmitted through the optical anisotropic diffraction grating by almost 100% and becomes circularly polarized by the retardation film. After that, the light beam passes through the aspherical lens (objective lens), is reflected by the recording surface of the optical recording medium, is transmitted through the aspherical lens again, and is transmitted through the phase difference film again.
It is converted into a wave and enters the optically anisotropic diffraction grating. The optical anisotropic diffraction grating functions as a diffraction grating for S waves.

For example, in the case where a rectangular concavo-convex portion which is bilaterally symmetric in a cross section perpendicular to its longitudinal direction and the depth thereof is appropriately set, in principle, a diffraction efficiency of about 40% in the first-order diffracted light direction, The light can be diffracted in the direction of the −1st order diffracted light with a diffraction efficiency of about 40%.

The cross-sectional shape of a plane perpendicular to the longitudinal direction of the concave-convex portion may be a symmetrical rectangular shape such as a rectangle or a square, or may be asymmetrical such as a step or a saw. In the case of a left-right asymmetrical shape, one of the ± 1st-order diffracted lights by the optically anisotropic diffraction grating has a higher diffraction efficiency, and only the diffracted light with the higher diffraction efficiency needs to be detected. Is preferable because high light utilization efficiency can be obtained.

Further, the uneven portion can be changed such that a distribution is given to the interval between the uneven portion, the symmetrical portion and the laterally asymmetrical portion are mixed, and the uneven portion and the convex portion are mixed. .

In view of the above-mentioned constitution and effect, the diffraction element of the present invention functions as a polarization beam splitter which is easy to integrate and has high efficiency. By using such a diffractive element, an optical pickup with high light utilization efficiency can be realized.

However, the depth of the above-mentioned grid-like uneven portion is
When the difference between the extraordinary index of refraction of the liquid crystal composition and the index of refraction of the substrate (which is almost equal to the ordinary index of refraction of the liquid crystal composition) multiplied by the depth of the irregularities is equal to half of the light wavelength, the The highest diffraction efficiency is obtained.

Therefore, in order to realize a high diffraction efficiency as a relatively shallow uneven portion which is advantageous in mass production, the difference between the refractive index of the substrate and the extraordinary refractive index of the liquid crystal composition, or substantially the ordinary light of the liquid crystal composition. It is necessary that the difference Δn between the refractive index and the extraordinary light refractive index be large.

On the other hand, in a liquid crystal composition material which is usually used in a liquid crystal display device or the like, when the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal composition is large, the liquid crystal composition has characteristics as a display device. It is not always preferred because it is sensitive to variations in the filled space (cell gap). for that reason,
As a liquid crystal composition for a liquid crystal display device, Δn is 0.20.
The following are commonly used:

In the present invention, since Δn is 0.21 or more, it is possible to form a relatively shallow concave-convex portion which is easy to mass-produce, and a desired high diffraction efficiency can be realized. Furthermore, Δn is 0.2 because a high diffraction efficiency can be realized in a shallower uneven portion.
7 or more is preferable. On the other hand, when Δn is larger than 0.36, the liquid crystal composition itself becomes unstable with respect to ultraviolet rays and the like, which causes a problem in reliability. Therefore, Δn is set to 0.35 or less.

Further, as the liquid crystal composition, a liquid crystal composition which can maintain a high refractive index difference in a wide temperature range and has a small fluctuation rate is preferable. A liquid crystal composition that exhibits liquid crystallinity in a wide temperature range is also preferable in relation to the variation rate. In the case of the diffractive element of the present invention, the operating temperature range is usually 0 to 60 ° C. Therefore, when the phase transition temperature from the nematic phase, which is a liquid crystal phase, to the isotropic phase, which is a non-liquid crystal phase, is 80 ° C. or higher, the fluctuation of the extraordinary refractive index is particularly small in the range of 0 to 60 ° C. Temperature changes can be reduced.

On the low temperature side, at a temperature of at least about 0 ° C., a phase from a nematic liquid crystal phase to a smectic liquid crystal phase or a solid liquid crystal phase is obtained in order to ensure stable operation and reliability during storage at lower temperatures. The transition temperature is preferably -10 ° C or lower.

The characteristics as described above, that is, high Δn,
A liquid crystal composition containing a compound represented by the following general formula in an amount of 60% by weight or more is preferable as a material that achieves a low temperature change rate, a wide temperature range of a nematic liquid crystal phase, and high reliability.

[0029]

Embedded image

However, A is a phenylene group (hereinafter referred to as -Ph
Abbreviated-) or trans-1,4-cyclohexylene group (hereinafter abbreviated as -Ch-), m is 0 or 1, X
Is a fluorine atom or a hydrogen atom, Y is a cyano group, a fluorine atom or a chlorine atom, Z is a fluorine atom or a hydrogen atom, R is a linear alkyl group having 2 to 8 carbon atoms or a linear alkoxy group having 2 to 8 carbon atoms. It is a base.

Further, 4'-trans-n-propyl-4-cyclohexyl-1-cyanobenzene, 4'-trans-n-propyl-4-cyclohexyl-1-fluorobenzene and the like are appropriately mixed in the liquid crystal composition. You may.

The diffraction element of the present invention may further have another diffraction grating formed on the surface on the light source side, in which case tracking error detection by the three-beam method is preferable.

The pattern of the concavo-convex portion (optical anisotropic diffraction grating) of the present invention has a curvature in the diffraction grating plane or a grating interval so that the beam shape of the return light from the optical recording medium becomes a desired shape. You can also give a distribution to.

In the optically anisotropic diffraction grating, two transparent substrates each having an uneven portion formed in a diffraction grating pattern on the surface are filled with a liquid crystal composition in the uneven portion facing each other. Alternatively, they may be formed by stacking. In that case, the depth of each concavo-convex portion may be shallow, which is preferable because it facilitates production. It is also preferable in that the orientation of the liquid crystal composition is improved by the two concavo-convex portions facing each other.

When the uneven portions formed on the two transparent substrates are laminated so as to be asymmetric with respect to the laminated surface, a diffraction grating having an asymmetric cross-sectional shape can be easily manufactured.
There is an effect that the diffraction efficiency of one of the ± first-order diffracted lights can be increased and the light with the higher diffraction efficiency can be detected by one detector, which is preferable.

When at least one of the light source side surface and the optical recording medium side surface of the diffractive element of the present invention is provided with a coating such as a UV curable acrylic resin, a λ / 4 plate or a glass substrate is formed. It is preferable since the wavefront aberration caused by the unevenness of the surface can be reduced. Further, by laminating a glass substrate or a plastic substrate having good flatness on the coating film of the UV-curable acrylic resin or the like, it is possible to significantly reduce the wavefront aberration, which is preferable. Therefore, since the light entrance / exit surface of the diffractive element is flattened, the wavefront aberration is consequently reduced.

As the light source of the present invention, a semiconductor laser, YA
Various solid and gas lasers such as solid-state lasers such as G lasers and gas lasers such as He-Ne can be used, and semiconductor lasers are preferable in terms of downsizing and weight reduction, continuous oscillation, maintenance and inspection. Further, by using a harmonic generator (SHG) in which a semiconductor laser or the like and a non-linear optical element are incorporated in the light source unit and a short wavelength laser such as a blue laser is used, high density optical recording and reading can be performed.

The optical recording medium in the present invention is a medium capable of recording and / or reading information by light. Examples of the optical recording medium include a CD (compact disc), a CD-ROM, a DVD (digital video disc) optical disc, a magneto-optical disc, and a phase change type optical disc.

[0039]

【Example】

Example 1 10 mm × 10 mm square × 0.5 mm thickness, refractive index 1.52
On the first glass substrate 1 of No. 3, a grid-like uneven portion 2 having a rectangular cross section with a depth of 1.2 μm and a pitch (cycle) of 10 μm was formed by photolithography and dry etching.

A second glass substrate 3 having a thickness of 10 mm × 10 mm × 0.5 mm and a refractive index of 1.52 was prepared, and a polyimide alignment film 5 was formed on the surface of the second glass substrate 3 in contact with the liquid crystal composition 4. The second glass substrate 3 was laminated and adhered to the first glass substrate 1 so that the rubbing direction of the polyimide alignment film 5 was along the stripe direction of the uneven portion 2.

The laminating and adhering of the two glass substrates was specifically carried out as follows. Epoxy resin 8 including a spherical spacer having a diameter of 4 μm was applied to the peripheral portion of the first glass substrate 1, the second glass substrate 3 was placed, and pressed from the upper portion to adhere. At this time, a region where the epoxy resin 8 was not partially applied was provided in the peripheral portion of the two glass substrates to form an opening for injecting the liquid crystal composition. At this time, the first glass substrate 1
The uneven portion (liquid crystal composition filled portion) 2 and the polyimide alignment film 5 of the second glass substrate 3 were faced to each other and bonded.

Then, in a depressurized atmosphere, the mixed liquid crystal composition "BL009" (a nematic liquid crystal composition manufactured by Merck,
Δn = 0.2915, ordinary refractive index = 1.5266, phase transition temperature to solid liquid crystal phase ≦ −20 ° C., phase transition temperature to isotropic phase = 108 ° C.) was injected through the opening. The opening was closed with a sealing resin to complete the sealing.

Polyimide alignment film 5 on second glass substrate 3
A retardation film (λ / 4 plate) 6 made of polycarbonate was adhered to the surface opposite to the surface using a transparent adhesive. Further, a UV curable acrylic resin is applied to the upper part of the retardation film 6, a third glass substrate 7 is pressed on the retardation film 6 and irradiated with ultraviolet rays to adhere the third glass substrate 7, and the diffractive element 10
Was prepared. A light incident surface (a surface of the diffractive element 10 opposite to the concavo-convex portion of the first glass substrate) 11 and a light emitting surface (third surface)
(The surface of the glass substrate opposite to the UV curable acrylic resin)
Reference numeral 12 denotes an antireflection film 1 for the light from the light source, respectively.
3 was formed.

As a result, the diffractive element 10 has a transmittance of 97% with respect to the P-wave having a wavelength of 678 nm (light having a polarization direction parallel to the paper surface in FIG. 1) from the semiconductor laser (not shown). It was The diffraction efficiency of the first-order diffracted light is 40.4% with respect to the S wave (light having a polarization direction perpendicular to the paper surface in FIG. 1) reflected from the optical disk (not shown).
Then, the diffraction efficiency of the −1st order diffracted light was 37.4%. Therefore, the reciprocating efficiency was 75.4%. The wavefront aberration of the transmitted light is due to the central portion (diameter 2 mm) of the light entrance / exit surface of the diffraction element.
It was 0.015 λ _rms (rms: root mean square) or less.

Example 2 Instead of the mixed liquid crystal composition of Example 1, a mixed liquid crystal composition (nematic liquid crystal composition, Δn = 0.2900, ordinary light refractive index = 1.5263, phase transition temperature to solid liquid crystal phase) ≤-2
(0 ℃, phase transition temperature to isotropic phase = 107 ℃)
Was injected through the opening. The opening was closed with a sealing resin to complete the sealing. As shown in Table 1, the mixed liquid crystal composition contains five kinds of compounds included in the general formula of Chemical Formula 2 as a main component (94% by weight). Other liquid crystal components include 6% by weight of alkoxycyanobiphenyl liquid crystal and alkylcyanoterphenyl liquid crystal.

As in Example 1, a polycarbonate retardation film (λ / 4 plate) 6 and a third glass substrate 7 were used.
Was adhered to form an antireflection film 13.

As a result of the above, the diffraction element 10 had a transmittance of 97% with respect to the P wave having a wavelength of 678 nm from the semiconductor laser. With respect to the S wave reflected from the optical disk, the diffraction efficiency of the first-order diffracted light was 40.2% and the diffraction efficiency of the -1st-order diffracted light was 37.1%. Therefore, the reciprocating efficiency was 75%. The wave aberration of the transmitted light was 0.015λ _rms or less at the center of the light entrance / exit surface of the diffraction element.

[0048]

[Table 1]

[0049]

Since the optical head device of the present invention uses a liquid crystal composition having a wide temperature range existing as a nematic liquid crystal phase and a large Δn, it is stable against temperature changes and has a shallow lattice shape. It has an effect that high light utilization efficiency can be realized in the uneven portion.

[Brief description of the drawings]

FIG. 1 is a side sectional view of an optical head device according to an embodiment of the present invention.

[Explanation of symbols]

 1: First glass substrate 2: Concavo-convex part 3: Second glass substrate 4: Liquid crystal composition 5: Polyimide alignment film 6: Phase difference film 7: Third glass substrate 8: Epoxy resin 10: Diffraction element 11: Incident surface 12: Emitting surface 13: Antireflection film

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location G02F 1/13 505 G02F 1/13 505 (72) Inventor Hiroki Hodaka Hazawa, Kanagawa-ku, Yokohama, Kanagawa 1150, Machi Asahi Glass Co., Ltd. Central Research Laboratory

Claims (4)

[Claims] 1. An optical head device for writing information and / or reading information by irradiating an optical recording medium with light from a light source through a diffractive element, wherein the diffractive element has a grid pattern on a surface of a transparent substrate. And an optical anisotropic diffraction grating filled with a liquid crystal composition having optical anisotropy. The liquid crystal composition has an ordinary wavelength at a temperature of 20 ° C. and an optical wavelength of 589 nm. A nematic liquid crystal composition having a difference Δn between the refractive index and the extraordinary light refractive index of 0.21 to 0.35 and a phase transition temperature from a nematic phase to an isotropic phase of 80 ° C. or higher. Optical head device. 2. The liquid crystal composition has a temperature of 20 ° C. and a light wavelength of 58.
The optical head device according to claim 1, wherein Δn at 9 nm is 0.27 to 0.35. 3. The optical head device according to claim 1, wherein the liquid crystal composition has a phase transition temperature from a nematic phase to a smectic phase or a solid liquid crystal phase of −10 ° C. or lower. 4. The optical head device according to claim 1, wherein the liquid crystal composition contains a compound represented by the following general formula in an amount of 60% by weight or more. Embedded image However, A is a phenylene group or trans-1,4-cyclohexylene group, m is 0 or 1, X is a fluorine atom or a hydrogen atom, Y is a cyano group, a fluorine atom or a chlorine atom, Z is a fluorine atom or a hydrogen atom, R is a linear alkyl group having 2 to 8 carbon atoms or a linear alkoxy group having 2 to 8 carbon atoms.